1. Field of the Invention
The present invention relates to an optical filter which selectively receives a signal of the desired optical frequency from a plurality of optical-frequency multiplexed light signals, a method of controlling the transmission wavelength of the optical filter, and an optical receiver using this method.
2. Description of the Related Art
Due to a recent significant increase in the amount of information, a demand exists for large-capacity communication systems. As a large-capacity communication system, an optical communication system is the most promising. The optical communication system of 2.488 Gb/s is used at present.
As the amount of information increases, optical communication systems with a larger capacity are demanded. The capacity of optical communication systems is increased by an increased transmission rate (TDM: Time Division Multiplexing), and multiplexing schemes, such as FDM (Frequency Division Multiplexing) and WDM (Wavelength Division Multiplexing).
The increase in transmission rate requires faster processing speed of electronic circuits on the transmitter and receiver sides; the limit of this speed is currently believed to be several tens of Gb/s.
The multiplexing schemes such as FDM and WDM, which use a wide band characteristic, can ensure a large capacity of several tens to several hundreds of Gb/s when used with a certain degree of improvement on the transmission rate. In addition, this system can easily execute multiplexing and separation using a photocoupler and an optical filter, reducing the burden on electronic circuits, and is thus promising.
FDM and WDM type optical receivers selectively receive a signal of a channel with the desired optical frequency from multiplexed signals. The easiest way to make that selection is to perform filtering in an optical region using an optical filter.
As there are various types of optical filters and various filter characteristics, the optical filter of a receiver should be designed to satisfy the system requirements and in consideration of practicality, simplicity, reliability, etc.
An example of the conventional wavelength multiplexing of light signals is the technique described in "Experiment on 1.55-nm Wavelength Multiplexed Transmission at 2.4 Gb/s Using An Optical fiber Amplifier" in Electronic Information Communication Committee, 1992 Spring Conference, Proceeding B-987.
This paper shows data on experiments of multiplexing two light signals with a coupler, sending the multiplexed signal to a transmission path and extracting from two filters signals from the multiplexed signal sent over the transmission path for reproduction on the receiver side.
This paper reports that the receiver could extract two signals with a wavelength interval of 8.8 nm from the respective filters with a difference of 16 dB.
As the wavelength interval in the prior art described in that paper is 8.8 nm, the disclosed art cannot be applied directly to cope with further multiplexing and further improvement on the transmission rate in the future.
Another prior art is disclosed in the magazine "Electronics Letters, 15th Mar. 1990 Vol. 26 No. 6". The second prior art concerns with optical FDM transmission of 100 channels. More specifically, this prior art multiplexes 100-channel light signals, and separates and reproduces those signals on the receiver side. The prior art has an optical filter constituted of seven stages of cascade-connected Mach-Zehnder interferometers, and employs a technique of controlling heaters provided on the respective Mach-Zehnder interferometers to tune the optical frequency to the desired level.
Although the prior art disclosed in the Electronics Letters accomplishes multiplexing and separation of 100 channels, its use of seven stages of Mach-Zehnder interferometers as an optical filter enlarges the apparatus and complicates the tuning control.
An optical filter which has a narrow-band transmission characteristic is an important element to remove ASE (Amplified Spontaneous Emission) noise from an optical amplifier besides a wavelength selecting filter in wavelength multiplexing/frequency multiplexing communication.
The narrow-band optical filter is conventionally constituted of a dielectric laminated film or a structure using an interferometers, such as a Mach-Zehnder interferometer or Fabry-Perot interferometer. Although the dielectric laminated film is structurally stable and has already been put to practice, it is difficult to attain a narrow-band transmission characteristic with a full width at half maximum of 1 nm or less.
The Fabry-Perot interferometer can easily attain a narrow-band transmission characteristic with a full width at half maximum of 1 nm or less, but is structurally unstable.
To eliminate the instability, a small Fabry-Perot interferometer using an optical fiber has been proposed. FIG. 22 shows the structure of this small Fabry-Perot interferometer using an optical fiber.
As shown in FIG. 22, the end faces of fibers are finished with mirror coating to become mirrors of high reflectivity, which are arranged with a certain gap therebetween to constitute an optical resonator. Further, PZT (Lead Zirconate Titanate) is used to control the transmission peak wavelength, thereby finely adjusting that gap. U.S. Pat. Ser. No. 4,861,136 also discloses an optical resonator with a similar structure.
This structure is relatively resistant to vibration due to its compactness and will not raise any problem when used steadily under the laboratory environment. But it has been observed that even light touching on the housing of this optical resonator greatly fluctuates the transmission peak wavelength.
Further, the adaptation of this optical resonator for use in actual communication systems is difficult because there are some doubts about its long-term reliability; for example, PZT is difficult to control due to its hysteresis, the reliability of PZT itself is not sufficient and it is difficult to stably keep the gap between the fibers with a movable portion attached.
Such unstable factors can be eliminated by a resonator constituted of a bulk member, such as quartz glass.
For example, parallel flat glass substrates may be coated with high-reflective films as shown in FIG. 23 to be used as an optical resonator. The structure shown in FIG. 23 has no movable portion so that high stability can be expected, with difficulty in controlling the transmission peak wavelength.
The narrow-band optical filter requires that the wavelength of the transmission light source should match with the transmission wavelength of the optical filter. A laser diode is generally used as the transmission light source. The laser diode also has an unstable factor.
The oscillation wavelength of the laser diode as a transmission light source has a large variation so that the wavelength probably changes due to a time-dependent change in the light source. The use of the laser diode in an optical communication system that uses a narrow-band optical filter requires automatic activation control to adjust the transmission wavelength of the optical filter to the wavelength of the transmission light source at the time the system is activated, and then cause the transmission wavelength range of the optical filter to follow up a change in the wavelength of the light source.
However, there have been no optical communication systems which use such a narrow-band optical filter or no techniques of permitting the transmission wavelength range of the optical filter to follow up a change in the wavelength of the light source.